%%%-------------------------------------------------------------------
%%% File    : p8.erl
%%% Author  :  Plamen Dragozov <plamen at dragozov.com>
%%% Description : 
%%% Find the greatest product of five consecutive digits in the 1000-digit number.
%%% 73167176531330624919225119674426574742355349194934
%%% 96983520312774506326239578318016984801869478851843
%%% 85861560789112949495459501737958331952853208805511
%%% 12540698747158523863050715693290963295227443043557
%%% 66896648950445244523161731856403098711121722383113
%%% 62229893423380308135336276614282806444486645238749
%%% 30358907296290491560440772390713810515859307960866
%%% 70172427121883998797908792274921901699720888093776
%%% 65727333001053367881220235421809751254540594752243
%%% 52584907711670556013604839586446706324415722155397
%%% 53697817977846174064955149290862569321978468622482
%%% 83972241375657056057490261407972968652414535100474
%%% 82166370484403199890008895243450658541227588666881
%%% 16427171479924442928230863465674813919123162824586
%%% 17866458359124566529476545682848912883142607690042
%%% 24219022671055626321111109370544217506941658960408
%%% 07198403850962455444362981230987879927244284909188
%%% 84580156166097919133875499200524063689912560717606
%%% 05886116467109405077541002256983155200055935729725
%%% 7163626956188267042825248360082325753042075296345
%%%
%%% Created :  2 Dec 2008
%%%-------------------------------------------------------------------

-module(p8).

%% API
-export([solution/2, test/0]).

%%====================================================================
%% API
%%====================================================================
%%--------------------------------------------------------------------
%% Function: solution(X, NumDigits) -> int()
%% Description:
%% Returns the greatest product of NumDigits consecutive digits in X.
%%--------------------------------------------------------------------
solution(X, NumDigits) ->
    product(X, NumDigits, 1, []).

test() ->
    solution(7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450, 5).

%%====================================================================
%% Internal functions
%%====================================================================
%Calculate the maximum product of N consecutive digits
product(0, _, Max, _) -> Max;
product(X, N, Max, Digits) when (length(Digits) < N) ->%The first N digits
    Next = X rem 10,
    product(X div 10, N, Max * Next, Digits ++ [Next]);
product(X, N, Max, [_|T]) ->
    L = T ++ [X rem 10],%A queue like structure would be more efficient, but not a big deal here
    Prod = lists:foldl(fun(Y, Acc) -> Y * Acc end, 1, L),
    NewMax = case Prod > Max of
                 true -> Prod; 
                 _ -> Max
             end,
    product(X div 10, N, NewMax, L).

